am62x: Assemble firmware image required to boot on HS-FS hardware

The am62x image script currently assembles an image for booting on early sk-am62 EVMs that where shipped with the general purose (GP) variant of the SoC. The latest generation of this EVM are shipping with High Security - Field Securable (HS-FS) SoCs that need different firmware. For now, just build this firmware as we don't currently have hardware to test with.

Signed-off-by: Martyn Welch martyn.welch@collabora.com